The very first superhero crossover, 1940.

Top: MLJ Comics house ad promoting the meeting of the the Shield and the Wizard, April 1940 cover date.
Middle: Pep Comics #4, May 1940 cover date.
Bottom: Top-Notch Comics #5, May 1940 cover date.

Why yes, their momentous meeting is a little underwhelming! Within months, though, Timely Comics—now known as Marvel Comics—would improve the art of the crossover considerably.

The difference between an unbeliever sinning and a Christian sinning is the difference between a man transgressing the laws of…[the] State, and … a husband [who] has done something he should not do in his relationship with his wife. He is not breaking the law, he is wounding the heart of his wife
—  D Martin Lloyd Jones commentary on Romans 7-8